1. Field of the Invention
The present invention relates to an image processing system and a method therefor, and more particularly to an image aberration compensation system for digital camera and a method therefor.
2. Related Art
The basic imaging principle of a digital camera (DC) is similar to that of a conventional camera. The DC and the conventional camera both have an optical lens set including at least one lens, which projects the light beams of an object to be shot onto a photosensitive material inside the camera according to an optical principle of pinhole imaging. The photosensitive material of a conventional camera is, for example, film (or referred to as negative film); while a DC adopts, for example, charge coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) as photosensitive materials (all-together referred to as photosensitive element below). The photosensitive element of a DC can convert the image shot by the optical lens set into digital signals, and the digital signals are stored in a storage medium (for example, a flash memory card or a built-in storage medium) after being processed by an electronic circuit. Different from the conventional camera, the image of the DC is stored in a digital mode, so the shot image can be modified through digital signal processing, for example, by a digital signal processor (DSP) to eliminate the noises of the image or adjust the contrast, chroma of the image.
The lenses adopted by the optical lens set of the DC are the same as those of the conventional camera, wherein the lens can be classified into, for example, spherical lens or aspheric lens. FIG. 1 is a schematic view of the aberration of a conventional spherical lens 110. Referring to FIG. 1, the light beam refractive angles at the periphery of the spherical lens 110 may deviate from the normal focal length along with the surface angles of the spherical lens 110 (i.e., cannot be concentrated on the focus). The distance between a focus Fc of the refracted light beams at the center of the spherical lens 110 and a focus Fb of the refracted light beams at the periphery of the spherical lens 110 is referred to as an aberration distance. In general, the aberration distance of a spherical lens varies with the surface angles of the spherical lens. The larger the aberration distance of a spherical lens is, the more apparent aberration the shot image may have. The so-called aberration refers to the problem of a shot image that the center of the image is distinct but the periphery (or a portion of the image) is obscure. Such problem is generally seen in optical lens set adopting a spherical lens. To alleviate the above problem, a method of, for example, using a plurality of spherical lenses to eliminate aberration can be employed. However, the overall transmittance, sharpness of such optical lens set and the contrast, definition of the shot image may be alleviated as the number of the spherical lenses is increased. In addition, the increase of the number of the spherical lenses represents the increase of the volume of the optical lens set and the raise of the fabrication cost of the optical lens set. Therefore, at present, a portion of the optical lens set is replaced by aspheric lenses to avoid the aforementioned aberration problem of a spherical lens. Being specially designed, the surface angles of an aspheric lens can make the center of the lens have the same focal length as the periphery region (i.e., imaging at a normal focal length). However, as the processing cost and technology of an aspheric lens are much higher than those of a spherical lens, the fabrication cost of the aspheric lens is much higher than that of the spherical lens. Moreover, the adoption of an aspheric lens still cannot solve the aberration problem caused by different object distances.